1. Field of the Invention
The present invention relates to a method and apparatus of measuring backward light, which measure and reduce light which is generated by the laser processing and which returns into a laser processing apparatus, and also relates to a laser processing method.
2. Related Background Art
In a light source device (laser processing apparatus) user in laser processing and the like, a laser beam emitted from a laser light source part is guided to an optical head (including an emitting optical system) via a light guide, and then radiated from the optical head to an object surrounded by the laser beam. Generally, a laser diode is used as the laser light source part, but sometimes a fiber laser having an amplifying optical fiber is used. In such a light source device, if reflected light (backward light) that is generated in the object irradiated with the laser beam returns to the laser light source part through the optical head and the light guide, the laser light source part might be damaged. To prevent such damage, it is desired to use an optical isolator for blocking the incidence of the backward light into the laser light source part.
The optical isolator is generally used on a single-lateral-mode beam. Such isolation is described in Japanese Patent Application Laid-Open No. H5-224151 (Document 1) and Examined Japanese Patent Application Publication No. S61-058809 (Document 2). Specifically, an isolator is defined assuming the case in which backward light from a reverse direction (backward beam) enters through the same optical path as an optical path through which a laser beam made incident in a forward direction (forward beam) passes after passing through the isolator. In this case, as described in Document 1 and Document 2, the backward beam is not terminated inside the isolator but generates differences in the emission position and emission angle from a light incident end of the isolator. Therefore, the backward beam is simply not joined to the laser light source generating the forward beam.
As disclosed in Document 1, when a birefringent element of the isolator is in a plate shape, only the emission position of the backward beam emitted from the light incident end of the isolator is changed. However, there is a limit to the shift of the emission position due to a birefringence ratio, and supposed use of a beam having a large mode field diameter might deteriorate the isolation itself.